I picked up a set of winter wheels and tires from the tire rack, and was able to fit them all like others posted. I doubt you'd be able to do it with only half of the rear folded down- one passenger only. The split fold down is spectacular, especially coming from a sedan with only a center pass through.

If your road trip takes you up north to snow, just ride with your winters on and don't worry about it. There are plenty of performance winters out there that do great on the freeway. I'm very pleased with my Michelin Pilot Alpin PA2 ZP Run Flats